1. Field of the Invention
The present invention relates to an automatic transmission.
2. Discussion of the Background
An automatic transmission capable of providing 8-forward-speed transmission using a first planetary gear set for input, second and third planetary gear sets for varying the speed, and six engagement mechanisms has been developed (refer to, for example, Japanese Unexamined Patent Application Publication No. 2005-273768).
According to Japanese Unexamined Patent Application Publication No. 2005-273768, the first planetary gear set for input is formed from a first sun gear, a first ring gear, and a first carrier that supports a pair of first pinions meshed with each other, one of which is meshed with the first sun gear and the other is meshed with the first ring gear. The first pinions can rotate about their own axes and revolve around the first sun gear. That is, the first planetary gear set is a so-called double-pinion planetary gear set. If the carrier is fixed, the sun gear and the ring gear rotate in the same direction. Accordingly, the planetary gear set is referred to as a “plus planetary gear set” or a “positive planetary gear set”. Note that if the ring gear is fixed, the sun gear and the carrier rotate in opposite directions.
In the first planetary gear set, the first sun gear serves as a fixed element that is fixed to a transmission case, the first carrier serves as an input element that is connected to the input shaft, and the first ring gear serves as an output element that reduces the rotational speed of the first carrier serving as the input element and outputs the rotation.
In addition, each of the two planetary gear sets for changing the speed is formed from a Ravigneaux planetary gear set including a second sun gear, a third sun gear, a second ring gear integrated into a third ring gear, and a second carrier that supports a pair of second pinions meshed with each other, one of which is meshed with the second sun gear and the second ring gear and the other is meshed with the third sun gear. The second pinions can rotate about their own axes and revolve around the second sun gear.
Assume that in a collinear diagram (a diagram indicating a ratio of the relative speed of one rotary element to the speed of another rotary element in the form of a straight line) of the Ravigneaux planetary gear set, a first rotary element, a second rotary element, a third rotary element, and a fourth rotary element are arranged in sequence at intervals corresponding to the gear ratios. Then, the first rotary element corresponds to the second sun gear, the second rotary element corresponds to the second carrier integrated into the third carrier, the third rotary element corresponds to the second ring gear integrated into the third ring gear, and the fourth rotary element corresponds to the third sun gear.
The engagement mechanisms are formed from a first wet multiple disc clutch that releasably connects the first ring gear formed from the output element of the first planetary gear set to the fourth rotary element formed from the third sun gear, a second wet multiple disc clutch that releasably connects the input shaft to the second rotary element formed from the second carrier, a third wet multiple disc clutch that releasably connects the output element formed from the first ring gear to the first rotary element formed from the second sun gear, a fourth wet multiple disc clutch that releasably connects the input element formed from the first carrier to the first rotary element formed from the second sun gear, a first brake that releasably fixes the first rotary element formed from the second sun gear to the transmission case, and a second brake that releasably fixes the second rotary element formed from the second carrier to the transmission case.
According to such a configuration, by engaging the first wet multiple disc clutch with the second brake, a first gear ratio is obtained. In addition, by engaging the first wet multiple disc clutch with the first brake, a second gear ratio is obtained. By engaging the first wet multiple disc clutch with the third wet multiple disc clutch, a third gear ratio is obtained. By engaging the first wet multiple disc clutch with the fourth wet multiple disc clutch, a fourth gear ratio is obtained.
Furthermore, by engaging the first wet multiple disc clutch with the second wet multiple disc clutch, a fifth gear ratio is obtained. By engaging the second wet multiple disc clutch with the fourth wet multiple disc clutch, a sixth gear ratio is obtained. By engaging the second wet multiple disc clutch with the third wet multiple disc clutch, a seventh gear ratio is obtained. By engaging the second wet multiple disc clutch with the first brake, an eighth gear ratio is obtained.
In addition, an existing automatic transmission includes eight trains along the axis line of the input shaft. More specifically, from a side of a torque converter, a first train formed from the fourth clutch and the first brake, a second train is formed from the first planetary gear set, a third train is formed from the first clutch, a fourth train is formed from the third clutch (note that although the third clutch seems to be in the same train as the first planetary gear set in a skeleton diagram, a piston and a hydraulic pathway for the third clutch are formed between the first clutch and the output gear, in reality), a fifth train is formed from the output gear, a sixth train is formed from the second planetary gear set, a seventh train is formed from the third planetary gear set, and an eighth train is formed from the second clutch and the second brake.